Broadcasting of television, radio, data and other forms of communications to receivers on mobile platforms using radio frequencies in the range from approximately 1-40,000 MHz suffer from transmission service outages. These transmission outages prevent the mobile user from receiving the broadcast and, if enough occur, create an unacceptable service. These outages are generally caused by multipath fading and reflection of the transmission path, physical blockage between the transmitter and receiver and interference to the mobile receiver from co-frequency transmitters other than the intended one.
Radio frequency transmission systems are subject to multipath fading which is caused by reflections of the desired signal from objects far from the mobile receiver such as mountains and from objects close to the mobile receiver such as a passing truck or large buildings. These reflections can either be specular or diffuse but are received along with the desired signal. Since the reflections are mostly out-of-phase with the desired signal, they either tend to cancel the desired signal or add to the receiver noise or both. When the cancellation and/or noise become sufficient with respect to the desired signal, a service outage occurs. See Reference Data for Engineers, 8th Edition, Prentice Hall, 1993, pp 46-10 for more details. Physical blockage between the transmitter and receiver is typically caused by the mobile receiver passing through tunnels, under overpasses or by close buildings and trees. During these blockage periods, whose durations are a function of the obstruction size/orientation and of the mobile receiver platform's speed, the desired signal is not available and an outage results.
Radio frequency broadcasting to mobile receivers is currently being implemented for satellite digital audio radio systems (see Report and Order, Federal Communications Commission, GEN Docket No. 90-357, Released Jan. 18, 1995 for more details) and has been used for many years for terrestrial radio including recently the inclusion of auxiliary digital data along with the audio program. It is possible in designing a radio broadcasting system to incorporate two or more transmission channels which transmit the same program information. Such multi-transmission channels may be provided for other purposes. Examples are transmission systems using spatial or frequency diversity for mitigating multipath. Alternatively, a multichannel transmission system can be purposely built to utilize time diversity. Hereafter the specification refers only to a two channel broadcast transmission system for simplicity. However, the same methods and apparatus described subsequently for two channel broadcast transmission systems are effective for three or more channel broadcast transmission systems.
The invention uses two transmission channels to provide time diversity for mitigating service outages from multipath, physical blockages and interference in mobile broadcast receivers. Each channel contains exactly the same data (e.g., program material). The program material in one of the channels is delayed at its origination source by a predetermined amount with respect to the second channel. The length of the delay is determined by the duration of the service outage which is to be avoided. At the mobile receiver, the transmission channel arriving first (i.e., with earlier data being the channel whose data were not delayed at its origination) is delayed using a storage type delay whose duration is the same as the duration of the delay in the data introduced at the origination point. The two received program channels are then combined or the program material in the two channels progressively selected by the receiver circuitry. The output of the combiner or selector in the mobile receiver always provides a time-ordered stream of all the program data even if there was a service outage in both transmission channels for a period equal to or less than the predetermined amount of delay introduced at the program origination point(s).
This invention is, in part, related to inventions disclosed and claimed in U.S. Pat. No. 5,278,863 which issued on Jan. 11, 1994 and U.S. Pat. No. 5,319,673 which issued on Jun. 7, 1994. Both patents are entitled "RADIO FREQUENCY BROADCASTING SYSTEMS AND METHODS USING TWO LOW-COST GEOSYNCHRONOUS SATELLITES". The contents of these two patents are incorporated herein by reference as though fully set forth here.